How to power an acoustic modem

I mentioned before that I’m basing my acoustic transducer off the Devantech SRF04 ultrasonic ranger. I’m removing the piezoelectric transducers from a dead SRF04 unit and using them for my modem. The transducers, which are the part that generate and receive the acoustic pressure wave that carries data, have a range of about 6 metres or so (one way), are driven with up to 20 V, and resonate at around 40 kHz.

The puzzle I’m working on right now is how best to actuate the transducer to generate a 40 kHz pulse. The SRF04 does it using a chip that’s intended to convert 5 V logic signals into the ±12 V signals used for the RS-232 serial protocol. Unfortunately most RS-232 converter chips aren’t made to power an acoustic transducer, and they aren’t able to provide enough current to generate a strong signal.

I bought a couple ADM208EANZ RS-232 converters, which seem like they should be able to provide up to 40 mA. I didn’t read the datasheet carefully enough and failed to notice that it needs a bunch of 0.1 μF polarized (i.e. aluminum electrolytic) capacitors. I have 0.1 μF ceramic caps, and I have various electrolytic caps, but I don’t have 0.1 μF electrolytic caps. The chip will generate +9 V with a 10 μF cap, but it won’t generate the -9 V rail. I have no idea why some circuits require capacitors with polarity, but if the diagram has a plus sign next to a capacitor you really need to pay attention. Ceramic capacitors, which don’t have polarity, might not work.

(Most RS-232 converter chips generate ±9 V instead of ±12 V, presumably because ±9 V is easier. The input is 5 V, then they put that through a voltage doubler to generate 10 V and put that through an inverter to generate -10 V. Then I guess they put both through something that drops 1 V (a regulator or something to stabilize the output) and you end up with ±9 V.)

If the ADM208EANZ can power the transducer, I think it will be the best solution. I see three downsides to using an RS-232 converter:

You’re limited to an 18 V swing (or perhaps even less) instead of the full 20 V. This isn’t a major problem because the transducer output is roughly a logarithmic function of the voltage input, so the output difference between 18 V and 20 V isn’t very high. But if you want to upgrade your transducer to something like the Maxbotix MaxSonar-UT transducer, which takes up to 60 V, then you’re stuck at 20 V.

The receiver’s amplifier is a little more complex because you have to operate it on a single rail (i.e. 0 V to 5 V instead of -10 V to +10 V). This downside is overwhelmed by the upside of not having to generate a ±10 V power supply.

The SRF04 documentation notes that they had to turn the RS-232 chip off while receiving to reduce noise. The noise is probably from the step-up converter and inverter that generate the positive and negative voltages. This is pretty annoying, and now that I mention it I recall that the ADM208EANZ doesn’t have a disable feature. I might be able to filter the noise.

The upside is that the RS-232 converter can be powered from the same 5V supply as the rest of the electronics. It doesn’t need a complicated battery assembly or external step-up converter, or a bunch of regulators to generate reliable voltage rails. It just needs a battery and one 5 V regulator, which is more power efficient and space efficient.